Background: Long ago at University I was a mainframe user and spent some time learning about the IBM and compatible systems the university had and a bit about their operating systems.  Later (but stll al long time ago) I was a long-time minicomputer user and sysadmin and had some exposure to higher-end minicomputers that were getting close to mainframe capabilities.

I started by giving quick reactions to the first bunch of example questions, but I think that's not what you want.  I left the reactions below, just in case they're useful.

Overall, a bunch of the questions seem like "knowledgeable-beginner-looking-for-more-understanding" questions - looking for an overview, some explanation of why things are the way they are.

i.e. not a good fit for StackExchange, the answers require almost a book.

As for why people who are interested in that stuff aren't participating - there just aren't enough enough.  Compared to the number of Windows Server or Unix/Linux servers, there just aren't a lot of mainframe systems around, so far fewer mainframe users/admins to (possibly) become interested in ServerFault or SE.  Also, since the ecosystem is smaller (lots of IBM stuff, a few other suppliers), I would think most of the expertise is "owned" by IBM and a few vendors.  If you want to know about TPF internals, you have to talk to IBM, there are no outside experts...  







Here are some quick judgements:  I wouldn't necessarily (and couldn't necessarily) defend these judgements, they're just my initial reactions...

>In z/OS assembler, what does it mean (and what do I have to do) when I’m told my program needs to be reentrant?

Looks like more of a programming question

> What is a dataset, and how is it the same, or different, than a file or directory on a *ix (Linux, Unix) system?

This would be considered too basic on many SE sites, the equivalent of "what's an inode in Linux"

> I just did an `apt install hercules`. How do I install an operating system on it?

Seems like too basic and too broad - How do I install Linux?  How do I install Windows...

> When JES2 complains of a SPOOL shortage, what’s the difference between TGS shortage, JQE shortage and JOE shortage?

Seems ok

> I’m having difficulty in a multi-step job Involving GDGs. I create a +1 data set in an early step but in a following step, I am getting “not found”.

On ServerFault, this would fall afoul of our "troubleshooting" close reason.

> What DISP parameter will let my batch job build a new loadlib while I have it allocated in TSOLIB?

Seems ok

> How can I find out what "RACF group" I'm in?

Don't know, maybe ok, maybe basic?

> What are the serialization mechanisms available in z/OS?

Seems like too basic and too broad - asking a broad question about OS fundamentals doesn't work well on SE

> Where can I get a free prebuilt operating system for an emulated IBM mainframe?
>
> I want to access MVS data sets using REST APIs ... how can I do that?

Both of these seem like shopping questions

> When would I use JCL and when would I use the Unix System Services (USS) shell to perform a task?

Maybe ok, but might be too opinion-based, like the difference between unix shells.

> I need to allocate a dataset. What does allocate mean, and what are record formats, logical record length, and number of tracks?

Another broad question about basics

> How do I access data sets from the USS shell?
> How can I create a Certificate Signing Request to install new certificates for web sites in a Sysplex?

Maybe ok

> Can I use my current open source tool chain for z/OS development? (It currently has Git, Jenkins, Ansible as the primary capabilities)

Too broad

> What are the fundamental differences between JES3 and JES2 ?

Maybe too broad, maybe ok... maybe there are a few really significant differences.

> Why is the FORCE command dangerous? Is it safer to use the similar functions provided in monitoring software etc?

Probably ok